1. Technical Field
The present invention relates to a display device, more particularly to a display device in which an ambient light photosensor that senses the brightness of ambient light is incorporated into a display panel, and a light source that illuminates the display panel is controlled according to an output of such ambient light photosensor.
2. Related Art
Over recent years, flat-type display panels have come into use not only in information and telecommunications equipment but in electronic equipment generally, and liquid crystal display panels are the most widely used among such panels. Liquid crystal display panels are provided with a backlight or the like that is lighted to illuminate the display screen when the ambient light is dim. Otherwise, the displayed images would be hard to view, since liquid crystals are non light-emitting. However, manual operation of the backlight is bothersome as it means having to turn the backlight on/off frequently according to the ambient brightness. Accordingly, technology has been developed (for example, JP-A-2006-243655) whereby the liquid crystal display panel incorporates an ambient light photosensor that senses the ambient light, and the backlight is turned on/off according to the results of such sensing.
FIG. 8 illustrates a known photosensing circuit and the signal waveforms generated thereby. This photosensing circuit LS is composed of a photosensing section Ls having an ambient light photosensor and an ambient light photosensor reading section Re that reads the photosensing section's output. Also, a thin film transistor (“TFT” below) ambient light photosensor Ts with a TFT as photosensing member is used for the ambient light photosensor of the photosensing section Ls. The operation of this photosensing circuit is as follows. A reverse bias voltage GV is applied in advance to the gate electrode G of the TFT ambient light photosensor Ts. With that state, first of all a switch S1 is turned on and a capacitor Cw is charged with a predetermined reference voltage Vs. Then the switch S1 is turned off, whereupon the charge accumulated in the capacitor Cw is discharged via the TFT ambient light photosensor Ts. When light exposes the TFT ambient light photosensor Ts, leakage current flows therein, and the leakage volume varies according to the quantity of light received. Therefore, the discharge is fast when the quantity of incoming light is large, and the discharge is slow when the quantity of incoming light is small. Thus, the charged voltage of the capacitor Cw falls with time according to the brightness, as indicated by the voltage waveforms for a terminal T2 in FIG. 8B. Accordingly, the output of the photosensing section Ls, in other words the voltage of the capacitor Cw, is compared with a predetermined reference voltage Vth, and if the voltage of the capacitor Cw is at or below the reference voltage Vth, the output of a comparator CP is inverted, the brightness of the ambient light is judged by determining the inversion time of such inverted output P, and lighting of a backlight (not shown) is controlled according to such judgment results.
When the photosensing circuit is incorporated into a liquid crystal display panel (not shown), however, the fact that various semiconductor parts such as TFTs for driving the liquid crystals, as well as drive circuits for electrodes and the like that link up such parts, are already deposed in large numbers on the substrates composing the liquid crystal display panel, means that parasitic capacitances between such drive circuits and the photosensing circuit's TFT ambient light photosensor's electrodes and wiring will be formed when the liquid crystal display panel is driven. Such parasitic capacitance will appear as, for example, parasitic capacitances Cvs and Cvd formed between the source electrode S and drain electrode D on the one hand and the common electrode Vcom on the other, plus parasitic capacitances Cs and Cd between the source electrode S and drain electrode D on the one hand and the gate electrode G on the other. When such parasitic capacitances occur, with a VCOM voltage constituted of rectangular waves, as illustrated by the VCOM voltage in FIG. 8B, normally being applied to the common electrode, if this VCOM voltage changes, that is, if it steps to high level H or low level L, mustache-like short pulses N1, N2 will arise in the output of the ambient light photosensor Ts under the attraction of such variation. These short pulses N1, N2 will be superimposed on the voltage of the capacitor Cw; more precisely, the positive short pulses N1 will be superimposed on the voltage of the capacitor Cw when the VCOM voltage is at the high level H, and the negative short pulses N2 will be superimposed on the voltage of the capacitor Cw when the VCOM voltage is at the low level L. However, these short pulses N1, N2 correspond not to the sensed value for ambient brightness, but to a kind of noise. Therefore, for example, superimposition of the short pulses N2 on the voltage of the capacitor Cw will cause such voltage to fall below a reference voltage Vth even though the capacitor Cw voltage value produced in response to the ambient light brightness does not reach the reference voltage Vth level. As a result, an output P of the comparator CP will be inverted, causing the backlight to be lighted erroneously, as indicated by the dot dash line K0 in FIG. 8B.